1. Field of the Invention
The present invention relates to a vehicle and a method for controlling engine start in a vehicle.
2. Background Art
Hybrid electric vehicles (HEV's) utilize a combination of an internal combustion engine with an electric motor to provide the power needed to propel a vehicle. This arrangement provides improved fuel economy over a vehicle that has only an internal combustion engine. One method of improving the fuel economy in an HEV is to shutdown the engine during times that the engine operates inefficiently, and is not otherwise needed to propel the vehicle. In these situations, the electric motor is used to provide all of the power needed to propel the vehicle. When the driver power demand increases such that the electric motor can no longer provide enough power to meet the demand, or if the battery state of charge (SOC) drops below a certain level, the engine must start quickly and smoothly in a manner that is nearly transparent to the driver.
One method of controlling an HEV powertrain is described in U.S. Pat. No. 6,176,808 issued to Brown et al. on Jan. 23, 2001, and incorporated herein by reference. Brown et al. describes an HEV powertrain that includes a clutch disposed between an engine and a motor, which is operable to disconnect the engine from the motor. The powertrain described in Brown et al. also includes a transmission located on an output side of the motor. The transmission includes a number of gears and clutches which allow the transmission to be operated at different speed ratios. A control method described by Brown et al. includes controlling the slip of reverse and forward clutches within the transmission during engine idle and vehicle launch. Brown et al. notes that during vehicle launch, the engine can be shutdown and the entire launch powered by the electric motor. In this situation, the forward clutch in the transmission is fully locked, and is not allowed to slip. Alternatively, Brown et al. notes that the engine can be kept running and the forward clutch in the transmission allowed to slip, thereby allowing the motor to assist the launch and provide optimum launch performance.
Starting an engine in an HEV can occur when the vehicle is operating under any one of a number of different conditions. Control of the engine start may be different for different sets of conditions under which the vehicle is operating. In addition, starting the engine in an HEV when it is moving under the power of the electric motor, may cause a noticeable, and therefore undesirable, torque disturbance in the vehicle driveline. Therefore, a need exists for a vehicle, and a method for controlling engine start in a vehicle, that reduces or eliminates driveline torque disturbances when the engine is started.